1. Field of the Invention
The present invention relates to an improvement of an apparatus for feeding various kinds of fluids such as gas for use in the manufacture of semiconductors, chemicals, precision machine parts, and the like. More particularly, this invention relates to a fluid feeding apparatus which permits high precision control of the flow rate of fluid at the times of starting to feed a fluid and of switching fluids from one kind to another.
2. Description of the Prior Art
Fluid feeding apparatuses requiring control with high precision of the flow rate are used in semiconductor manufacturing facilities and chemical production plants. Most of those apparatuses are equipped with so-called mass flow controllers.
FIG. 8 shows an apparatus for feeding fluid (gas) for use in a high purity moisture generator in semiconductor manufacturing facilities. It is designed so that H.sub.2 and O.sub.2 are fed to a reactor 51 from a gas feeding apparatus 50 at specific flow rates and are radicalized with a platinum catalyst and allowed to react with each other in a non-combustion state to generate moisture gas (water vapor). The moisture gas thus generated in the reactor 51 is then supplied to an oxidizing furnace 52.
In FIG. 8, the reference numeral 54 designates a circuit for measuring moisture generation responsiveness of the reactor 51, and includes a suction rate regulating valve 55, a quadrupole mass spectrometer (Q-mass spectrometer) 56, a turbo molecular pump 57, and a vacuum pump 58.
The Q-mass spectrometer 56 is for measuring ion concentrations of H.sub.2 O, H.sub.2, O.sub.2, and N.sub.2. A quadrupole mass analyzer MSQ-150A (ULVAC Corporation, Japan) is used for the purpose. The aforesaid gas feeding apparatus 50 is formed from three mass flow controllers MFC.sub.1, MFC.sub.2, and MFC.sub.3, changeover valves V.sub.1, V.sub.2, and V.sub.3, gas storage containers (not shown), and pressure regulators (not shown). The changeover valves V.sub.1, V.sub.2, and V.sub.3 in this example are of the electric metal diaphragm type.
From the respective gas storage containers (not shown), H.sub.2 at a gauge pressure of 2 kgf/cm.sup.2, O.sub.2 at a gauge pressure of 2 kgf/cm.sup.2, and N.sub.2 at a gauge pressure of 6 kgf/cm.sup.2 are supplied to the primary sides of the three mass flow controllers MFC.sub.1, MFC.sub.2, and MFC.sub.3.
To generate moisture in the reactor 51, in the meantime, the flow rates and other conditions of the three mass flow controllers MFC.sub.1, MFC.sub.2, and MFC.sub.3 in the gas feeding apparatus 50 are set, and then the system is purged with N.sub.2, V.sub.1 and V.sub.2 being closed and V.sub.3 being opened. Then V.sub.3 is closed and V.sub.2 , is opened to supply O.sub.2, and at the same time that O.sub.2 is supplied, or about 3 seconds after the O.sub.2 supply, V.sub.1 is opened to supply H.sub.2. Thus, moisture gas (water vapor) starts to be generated in the reactor 51.
Part of the moisture gas or the like from the reactor 51 is sucked into the measurement circuit 54 for a specific time by operating the suction-regulating valve 55, where the concentrations of H.sub.2, O.sub.2, H.sub.2 O, and N.sub.2 in the generated moisture are measured by the Q-mass spectrometer 56.
FIGS. 9 to 11 illustrate the concentrations of H.sub.2, O.sub.2, N.sub.2, and H.sub.2 O measured by the Q-mass spectrometer 56 in the moistures produced in a moisture-generating testing arrangement. The testing arrangement was provided with a gas feeding apparatus 50 equipped with the mass flow controllers as shown in FIG. 8. And the measurements were taken under the following conditions (1), (2), and (3). The gauge pressure of H.sub.2, O.sub.2, and N.sub.2 on the primary sides of the mass flow controllers in the gas feeding apparatus 50 were 2 kgf/cm.sup.2, 2 kgf/cm.sup.2, and 6 kgf/cm.sup.2, respectively.
(1) Pressure on the secondary side of the mass flow controllers: 1 kg/cm.sup.2 abs PA0 H.sub.2 : 50 sccm+O.sub.2 : 1000 sccm PA0 N.sub.2 : 1000 sccm PA0 (2) Pressure on the secondary side of the mass flow controllers:0.5 kg/cm.sup.2 abs PA0 H.sub.2 : 50 sccm +O.sub.2 : 1000 sccm PA0 N.sub.2 : 1000 sccm PA0 (3) Pressure on the secondary side of the mass flow controllers: 0.2 kg/cm.sup.2 abs PA0 H.sub.2 : 50 sccm+O.sub.2 : 1000 sccm PA0 N.sub.2 : 1000 sccm PA0 (1) Pressure on the secondary side of the mass flow controllers: 0.5 kg/cm.sup.2 abs PA0 H.sub.2 : 100 sccm+O.sub.2 : 50 sccm (H.sub.2 :O.sub.2 =2:1) PA0 N.sub.2 : 1000 sccm PA0 (2) Pressure on the secondary side of the mass flow controllers: 0.5 kg/cm.sup.2 abs PA0 H.sub.2 : 100 sccm+O.sub.2 : 50 sccm (H.sub.2 :O.sub.2 =2:1) PA0 N.sub.2 : 1000 sccm
H.sub.2 starts to be fed 3 seconds after O.sub.2 supply.
H.sub.2 starts to be fed 3 seconds after O.sub.2 supply.
H.sub.2 starts to be fed 3 second after O.sub.2 supply.
It is to be understood that "sccm" is a unit indicating the flow rate/minute in volume (cm.sup.3) of H.sub.2, O.sub.2, N.sub.2, etc. in the standard state. As is evident from FIGS. 9 to 11, the concentration of H.sub.2 rises and peaks at a peak P.sub.H2 at the start of the gas feeding as the pressure decreases on the secondary side of the mass flow controllers MFC's (pressure reduction) in the moisture generation testing arrangement provided with the gas feeding apparatus 50 equipped with mass flow controllers. Along with that, there appears a peak P.sub.H2 O in the concentration of H.sub.2 O.
H.sub.2 and H.sub.2 O peak at P.sub.H2 and P.sub.H2O in the initial stage of the gas feeding. That means it is impossible to precisely effect H.sub.2 concentration control (flow rate control). The mass flow controller cannot meet the demand for high precision control of the flow rate of H.sub.2.
It should also be noted that if the peak P.sub.H2 of the concentration of H.sub.2 rises to several percent, there will arise a possibility of hydrogen exploding in the downstream oxidizing furnace 52, raising a safety problem.
On the other hand, FIGS. 12 to 13 also show the concentrations of H.sub.2, O.sub.2, N.sub.2, and H.sub.2 O measured by the Q-mass spectrometer 56 in the moistures produced in a moisture-generating testing arrangement. The testing arrangement was provided with a gas feeding apparatus 50 equipped with the mass flow controllers as shown in FIG. 8. And the measurements were taken under the following conditions (1) and (2). The supply pressures (gauge pressures) of H.sub.2, O.sub.2, and N.sub.2 on the primary sides of the mass flow controllers were 2 kgf/cm.sup.2, 2 kgf/cm.sup.2, and 6 kgf/cm.sup.2, respectively.
H.sub.2 and O.sub.2 started to be fed simultaneously and were cut off at the same time.
H.sub.2 started to be fed 3 seconds after O.sub.2 supply and the feeding of H.sub.2 was cut off three minutes earlier than O.sub.2 cut-off.
As is clear from FIG. 12, the concentration peak P.sub.H2 of H.sub.2 in the initial stage of gas feeding rises to some 10 percent if H.sub.2 and O.sub.2 are fed simultaneously in the gas feeding apparatus 50 equipped with the prior art mass flow controller and there arises a safety question.
Furthermore, the concentration P.sub.O2 of O.sub.2 suddenly falls in the region where a peak P.sub.H2 of H.sub.2 is observed because O.sub.2 is consumed in its reaction with H.sub.2. As a result, it will be impossible to generate an intended quantity of moisture.
Also, as is shown in FIG. 13, the concentration peak P.sub.H2 of H.sub.2 increases to more than some 50 percent at the beginning of gas feeding in the gas feeding apparatus 50 using the prior art mass flow controller, further increasing the danger.
In addition, a large quantity of O.sub.2 is consumed at the aforesaid peak P.sub.H2 of H.sub.2, resulting in a substantial fall in the concentration O.sub.2. That makes it difficult to produce a required amount of moisture.
As set forth above, the gas feeding apparatus 50 equipped with the prior art mass flow controllers has a problem that the flow rate of H.sub.2 and O.sub.2 is impossible to control with high precision, because H.sub.2 and O.sub.2 flow in excessively in what is called an overshooting in the initial stage of the gas feeding or suspension.
In generating moisture in the gas feeding apparatus 50 with the mass flow controllers, it is natural that the amount of moisture generated deviates greatly from the set level, because of the overshooting of H.sub.2 and O.sub.2, and it is difficult to control the generation of moisture with high precision.
In the meantime, in the manufacture of semiconductors or the like, it is often necessary to feed many different kinds of gases at specific flow rates to specific places or switch the feeding from one kind to another. Also, it is necessary to supply to specific places high purity water produced through a reaction between H.sub.2 and O.sub.2. It is always required that the supply or switchover of gases and H.sub.2 O should be effected quickly with high precision. That is because high precision control of the flow of gases and moisture is indispensable to improving or securing the production yield as well as the quality of finished products or semiconductors.